A common use case for UpKeep is that someone sees a broken piece of equipment, pulls out their phone, opens up UpKeep, snaps a picture, and sends it off to the technician for repair. The technician now has a prioritized list of his/her tasks for the day and can easily follow up with requests!
In terms of how did I both build and market with a full time job... I didn't do any marketing for UpKeep in the beginning. I had the most common misconception that "If I built it people would come". In the beginning, UpKeep was a free application for everyone, and I viewed it more as a hobby as I was learning to program. It slowly started gaining popularity in the "free to use" category for business applications and it was fueled all by what a cool app that's completely free.
Now... If you ask about the transition from a free product into a paid service... I felt so so bad doing it because we actually upset a lot of users during that transition. But yeah that's a whole new story :)